1. Alterations in dopamine neurotransmission have been implicated in a number of neurological conditions including Parkinson's disease, schizophrenia, attention deficit hyperactivity disorder, and drug addiction. The nuclear orphan receptor Nurr1 is essential for the terminal differentiation of midbrain neurons as we and others showed. The arrest of dopamine neuron precursors in development, by disruption of the Nurr1 gene by homologous recombination in mice, prevents expression of dopamine neuron specific proteins leading to the complete inhibition of neuron transmitter dopamine synthesis. We have identified some genes whose expression is affected by Nurr1. It appears that Nurr1 can function either as a repressor or inducer of the expression of specific genes. Using comparative microarray analysis of RNAs from wild type and Nurr1-null mice prepared from the ventral tegmental area has shown a large decrease in guanosine triphosphate cyclohydrolase (GTPCH) mRNA in Nurr1-null pups, which led to concomitant reduction in BH4 content. Microarray analysis showed 70% reduction in GTPCH expression in the ventral tegmental area of both 12.5-day old Nurr1-null embryos and neonates. Although levels of GTPCH mRNA increased significantly between E12.5 and birth in wild type mice, no such change was seen in the null neonates. The promoter deletional analyses of Nurr1 are underway to identify regions involved in Nurr1 mediated transcription of GTPCH in neuronal cells. 2. Recently, we have studied the role of Nurr1 in osteoblast differentiation. Our results revealed that reduced Nurr1 expression, using Nurr1 siRNA, affected the expression of osteoblast differentiation marker genes; osteocalcin, type one collagen and the activity of alkaline phosphatase. In addition, the expression of those genes was decreased in primary cultured mouse calvarial osteoblasts derived from Nurr1-null mice. Thus, our results suggest that Nurr1 is important for osteoblast differentiation. 3. We have also decided to investigate whether Nurr1 is involved in differentiation of other cell types. Understanding mechanisms involved in regulation of body weight is a very important goal due to the worldwide obesity epidemic. Recently, we discovered that Nurr1-null heterozygous mice have twice as much fat mass when compared to wild type mice when fed with either normal or high fat diet without significant differences in food intake. By hormonal induction, mouse embryonic fibroblasts (MEF) generated from Nurr1-null mice have shown increased rate of adipocyte differentiation as extimated by accumulation of oil droplets in contrast to MEF prepared from wild type littermates. When Nurr1 was constitutively overexpressed, using retroviral infection in MEF-Nurr1-null, the rate of adipogenesis was comparable to that observed in MEF wild type. In agreement with these results, the level of molecular markers of adipogenesis C/EBP alpha and PPAR gamma was increased in the absence and decreased in the presence of Nurr1, respectively. These results suggest that Nurr1 functions as a repressor of adipogenesis in vivo and in vitro and might play a role in adipogenesis differentiation linked to obesity.